This invention relates to MOSFETs (Metal Oxide Silicon Field Effect Transistors), and in particular to MOSFETs used in synchronous rectifier applications having very high currents and low voltages. In as much as the stored energy in an inductance increases as the square of the current, and in as much as the rate that current can change (di/dt) decreases in an inductance as the voltage is reduced, it is particularly important to minimize the parasitic inductance in low voltage, high current circuits such as synchronous rectifier circuits used for processor power supplies and the like.
Package inductance, and the inductance in the circuit board and other connections to MOSFETs are critical in low voltage, high current synchronous rectifiers. Some new MOSFET packages have very low inductance, yet even that low inductance will be too much as transformer frequencies increase to the megahertz range. In circuits with high di/dt, high frequency effects, especially the skin effect (penetration depth) are serious problems.
Many synchronous rectifier circuits use a pair of MOSFETs connected in a common source configuration. This configuration necessarily means that the drains of the MOSFETs must be separate and connected to the transformer (or other circuits) separately.